: The goal of this project is to demonstrate and validate that labeled tyrosine kinase inhibitors can be used as imaging agents for tumors overexpressing these receptor associated enzymes. The applicants have outlined a paradigm to develop unique positron and single photon emitting radiopharmaceuticals using the epidermal growth factor receptor tyrosine kinase (EGFRtk) and breast cancer as model systems. Many members of the receptor tyrosine kinase family are involved in tumor cell signal transduction pathways and are useful prognostic indicators. EGFR is overexpressed in 45 percent of breast tumors. Its prevalence has been correlated with poor responsiveness to hormonal therapy and poor patient prognosis. A series of aminoquinazoline derivatives, structurally similar to those know to inhibit EGFRtk at pico- to nano- molar concentration suitable for labeling with fluorine-18, carbon-11, bromine-76 or iodine-123 will be synthesized. The ability to inhibit EGFRtk activity will be tested in an in vitro whole cell autophosphorylation assay. Specificity will be determined by testing the quinazolines ability to inhibit erbB-2 and erbB-3 tyrosine kinase activity. Potent inhibitors will be subsequently radiolabeled with no carrier added fluorine-18, carbon-11 or bromine-76 produced in their cyclotron or with iodine-125 from Amersham. The accumulation of the labeled tracers in tumor cells (MDA-435, MCF-7, MDA-231, MDA-568) possessing various levels of EGFR expression will be assessed in vitro cell culture experiments; specific and non-specific binding will be measured. The amount of EGFR in the cell lines will be determined by a separate method and correlated with the uptake. The distribution and metabolic characteristics of the selected radiotracers will be evaluated in tumor-bearing mice. Tumor cells from the same cell lines used in the cell culture experiments will be implanted and grown in mice. Tracer distribution, efficacy and receptor-mediated uptake will be determined in the mouse model. Again, uptake in the tumors will be correlated with a direct measure of EGFR titer. Demonstration of the ability to visualize breast cancer with EGFRtk imaging argents will have dual impact. Not only will this expand our armamentarium of agents for the diagnosis and staging of breast lesions but it will also validate the use of tyrosine kinase-based imaging agents to target tumors in other tissues. Additionally, it will provide inroads into new areas of research in drug discovery and therapeutic intervention.